1. Field of the Invention
This invention relates to the white balance correction device of an image sensing apparatus such as a video camera or the like.
2. Description of the Related Art
The white balance correcting methods for the image sensing apparatus such as a video camera or the like can be roughly divided into two classes. One is a method of using a color temperature sensor. The other is a method of using a video signal.
FIG. 1 of the accompanying drawings is a block diagram showing the circuit arrangement of the conventional image sensing apparatus adopting the automatic white balance correcting method of using a video signal. Referring to FIG. 1, the illustration includes an image sensor 1; a luminance and chromaticity forming part 2; a red (R) signal gain control part 3; a blue (B) signal gain control part 4; a color-difference signal forming part 5; an encoder 6; gate parts 7 and 8; clipping parts 9 and 10; an R-B signal detecting part 11; an averaging part 12; a comparing and amplifying part 13; a tracking correction part 14 which is arranged to generate tracking correction signals. Parts 7-14 comprise and an automatic white balance correction signal generating device 15b.
Referring to FIG. 1, an image formed on the image sensor 1 is converted into an electrical signal. The electrical signal is supplied to the luminance and chromaticity forming part 2. The part 2 then forms a luminance signal YH which has a luminance signal frequency band, a luminance signal YL which has a color signal frequency band, and color signals including an R signal and a B signal respectively. The R and B signals are respectively supplied to the gain control parts 3 and 4. At the gain control parts 3 and 4, the levels of the R and B signals are controlled according to control signals output from the tracking correction part 14. These signals are then output from the parts 3 and 4 as an R' signal and a B' signal respectively. The R' and B' signals are then supplied together with the luminance signal YL to the color-difference signal forming part 5. At the part 5, color-difference signals R-YL and B-YL are formed from these three input signals. Further, the color-difference signals R-YL and B-YL are supplied together with the luminance signal YH to the encoder 6. As a result, the encoder 6 produces a standard TV signal.
The color-difference signals R-YL and B-YL are distributed via terminals M and N also to the automatic white balance correction signal generating device 15b. Within the device 15b, these input signals R-YL and B-YL are supplied to the gate parts 7 and 8. The gate parts (or circuits) 7 and 8 remove, from these inputs, unnecessary signals obtained during a blanking period and any abnormal signal that is caused by level saturation occurring in taking a picture of a high luminance object.
Signals R-YL and B-YL thus output from the gate parts 7 and 8 are supplied respectively to the clipping parts 9 and 10. At the clipping parts 9 and 10, any color-difference signal that has, within a practical color temperature range, an excessive degree of saturation is thus processed to limit its amplitude.
The color-difference signals R-YL and B-YL are thus subjected to the processes of varied kinds to be converted into signals (R-YL)' and (B-YL)'. The color-difference signals (R-YL)' and (B-YL)' are supplied to the R-B signal detecting part 11. At the part 11, an (R-B) signal is obtained from a difference between the signals R-YL)' and (B-YL)'. The (R-B) signal is supplied to the averaging part 12 to undergo an averaging process. As a result, the (R-B) signal of a value averaged for a whole image plane is obtained in a DC value. The averaged (R-B) signal is supplied to the comparing and amplifying part 13. At the part 13, the signal output from the averaging part 12 is compared with a reference voltage Vref1, which is a voltage corresponding to R-B=0. Then, a signal representing the result of comparison is supplied to the tracking correction part 14.
The tracking correction part 14 forms gain control signals for the gain control parts 3 and 4 on the basis of the signal output from the comparing and amplifying part 13. As a result, the circuit arrangement operates to correct the white balance of a video signal. As apparent from the above description, a negative feedback loop is formed by the gain control parts 3 and 4, the color-difference signal forming part 5 and the automatic white balance correction signal generating part 15b. The color-difference signals are supplied to the encoder 6 after white balance adjustment. The encoder 6 then produces a standard TV signal.
The conventional white balance correction device, however, sometimes makes a correction error. In a typical example of such cases, the color distribution of an object to be shot consists of 50% in white and 50% in yellow. The device operates in this case as described below with reference to a vectorial representation of FIG. 2 in conjunction with the block diagram of FIG. 1.
The points of white and yellow on the vectorial representation of FIG. 2 are assumed to be W and Ye when the gains of the gain control parts 3 and 4 of FIG. 1 are at the rate of one-fold. In this instance, the negative feedback loop acts to cause the (R-B) signal to be zero. Therefore, the direction of white balance correction is in parallel to an axis R-B of the vectorial representation. Points on vectors where the negative feedback action becomes stable are Ye' and W' at which there obtains a relation Ye-Ye'=Ye'-a=a-b=W-W'. A segment Ye-b is in parallel to the axis R-B. A point "a" is located on a segment which passes through an origin W and is perpendicular to the axis R-B.
As shown in FIG. 2, points Ye and W after correction are located in coordinate positions Ye' and W' respectively. In other words, the yellow color changes toward green and the white color toward blue. The white balance correction is thus not adequately performed.
As described above, in cases where the color temperature distribution of the object is uneven and eccentric, the conventional white balance correction device makes an error in correcting white balance. It has been thus difficult to attain an adequate effect of correction under such a condition.